1. Field of the Invention
The present invention relates to an improvement in wax master tree used in the lost wax casting process.
2. Prior Art
In the investment or lost-wax casting process, a so-called tree or cluster is used. The tree for this purpose is made by molding a plurality of masters for the desired products from a wax and attaching a pre-set number of wax masters to the outer peripheral wall of a sprue rod made from a wax.
However, since the sprue rod of conventional casting tree is made from a wax in its entirety, a relatively large quantity of wax is required for the production of tree. A more serious problem of the conventional casting tree is that the sprue rod of solid wax is not so strong as to ensure easy handling of the tree. Care must be paid when the tree is transported or conveyed by grasping the neck portion of the sprue rod, leading to reduction in efficiencies during the transportation and conveyance operations. Furthermore, a skillful operation is required for mounting the tree on an apparatus for applying the ceramic shell coating layer.
Another disadvantage of the conventional tree is that the ceramic shell coating, which is formed by coating a refractory material on the tree followed by drying, tends to be cracked by the action of thermal expansion of the wax during the dewaxing step of heating to melt the wax masters contained in the thus formed ceramic shell coating unless careful consideration is paid to avoid the adverse influence of thermal expansion. This problem, i.e. cracking of the ceramic shell coating layer caused by the thermal expansion of wax, is a troublesome one particularly in the ceramic shell mold process wherein the shell coating layer is not supported by a back-up mass. In order to solve this problem, it has been a common practice to heat the tree rapidly so that the boundary surface portions of the wax masters contacting with the interior surface of the ceramic shell coating are immediately melted to flow out of the ceramic shell coating layer before the whole mass of the wax masters is heated to increase the volume thereof by thermal expansion to an extent for causing the cracking problem. However, the cracking problem caused by thermal expansion has not yet been solved even by skillful heating, since a large amount of molten wax flow is formed by the molten sprue rod which is made from a wax in its entirety.